This invention is related to a method and apparatus for controlling VOC emissions from wood-product processing and manufacturing plants. More particularly, the invention is related to controlling VOC emissions during the drying of wood particles prior to their further processing into engineered wood products. In another aspect, the invention is related to efficiently utilizing the thermal energy generated during the manufacturing process.
Oriented strand board (OSB) is manufactured by first debarking the logs, and then breaking or xe2x80x9cwaferizingxe2x80x9d the wood into relatively small, thin wafer or strand like particles. The wood wafers are then dried. During the drying of wafers, volatile organic compounds (VOC""s) are also emitted from the wood particles into the drying air stream. The emitted VOC""s are entrained in the large volumes of heated air fed into the wafer dryers, and in air which is extracted from the workspaces in certain areas of the plant.
Current environmental regulations require containment and destruction of nearly all of the VOC""s emitted during the drying of the wood particles. The containment and destruction of the VOC""s is very expensive, both in terms of capital costs and operating costs. The high cost of controlling the VOC""s is due primarily to the large volumes of air that must be treated, rather than the overall amounts of VOC""s emitted. Containment and control of VOC""s is currently achieved by the use of large thermal reactors known as Regenerative Thermal Oxidizers (RTO""s). RTO""s burn a fuel (natural gas) to generate the high temperatures necessary to destroy the VOC""s. Multiple RTO""s are normally used, and are expensive to build, operate and maintain. As a result, RTO""s represent a sizable fraction of the initial cost of a new plant, and of the ongoing operating expenses associated with an OSB plant.
Turning now to FIG. 1, a typical OSB manufacturing process is shown in greater detail. Green wafers are transferred from green bin 10 into dryer 12 where the green wafers are dried from 100% of their green moisture content (MC) down to about 4-7%. The dried wafers and VOC-laden gas stream exit the drier 12 and are separated in cyclone 14. The dried wafers and fines are separated from the gas stream. The gas stream is sent to wet electrostatic precipitator 16 where the fine particulates are removed, and then RTO 18 where the VOC""s are thermally oxidized and destroyed before the gas stream is discharged to the atmosphere. In another section of the facility, VOC""s emitted from the press vent 20 are collected from the surrounding area in a relatively large volume air stream as discussed above, and introduced into a second RTO 22 where the VOC""s are destroyed.
In other known methods of controlling VOC""s, all or part of the drying air stream is recycled to a high temperature burner where the VOC""s are destroyed. EP 0 457 203 discloses a method wherein a major portion of the drying air stream is continuously recycled within the dryer. A second portion is continuously separated from the recycled drying air and is fed to a condenser where the high boiling components, including some VOC""s, are removed. The remainder of the stream is then introduced into a burner where any remaining hydrocarbons are destroyed. The VOC containing liquid generated in this method must be treated, which is difficult to achieve in typical biological sewage treatment plants. Another known method that is taught in EP-A-O 459 603 is similar, except that the condensation step is omitted. A portion of the recycled drying air stream is separated and fed directly into a burner where the hydrocarbons are destroyed. Each of these methods, while purporting to limit VOC emissions, requires the use of heat exchangers to transfer heat from the combustion stream to the drying air stream. In each of these methods, combustion gases at about 900 degrees F. are fed into a heat exchanger to heat the drying air stream to about 500 degrees F. In the portion of the heat exchanger where the combustion gases are introduced, the drying air stream is at about 500 degrees F. The heat exchanger suffers rapid degradation in those areas due to the high temperatures.
A prior art method shown in U.S. Pat. No. 5,697,167 to Kunz, et al attempts to address this problem and reduce the stress on the heat exchanger. As with the methods described above, the drying air stream is recycled with a small portion being separated and fed into the burner. In this method however, the recycled portion and the combustion gases are first introduced into a supplemental heat exchanger where the combustion gases are partially cooled and the recycled drying air stream is partially heated. Since the maximum temperature of the recycled drying air is lower, the heat exchanger runs cooler, extending the life of the heat exchanger. The combustion gases and the drying air stream are then introduced into a main heat exchanger wherein the drying air stream is heated to about 500 degrees F. as before. However, the combustion gases are partially cooled, resulting in a lower maximum temperature in the heat exchanger. In this way, the heat-induced stress on both heat exchangers is reduced. In the supplemental heat exchanger, the lower exit temperature of the drying air stream serves to cool the heat exchanger in the area where the combustion gases are introduced. In the main heat exchanger, the lower inlet temperature of the combustion gases results in a lower maximum temperature in the heat exchanger.
This method, while an improvement over the earlier methods, nonetheless has major limitations. First, an additional supplemental heat exchanger is required. Even though the lower temperatures extend the lives of the supplemental and main heat exchangers, the heat exchangers still represent a major capital and operating expense. Second, this method""s efficiency is limited by the maximum practical combustion gas temperature. As mentioned, the heat exchangers are degraded under conditions of inlet gas temperatures of about 900 degrees F. The temperature limitations of the heat exchangers aside, the maximum temperature of combustion gas stream is limited to about 1100 degrees F. Higher temperatures cause slugging problems in the heat exchanger, which result in significantly higher operating expenses. Slagging occurs when the combustion gas temperature is high enough to melt salts in entrained in the combustion gases. The molten salts then deposit and solidify on the cooler heat exchanger surfaces, causing plugging and reducing the heat transfer efficiency of the heat exchanger.
Applicants have discovered a novel method of drying the green wafers or other wood particles which reduces the volume of air in which the VOC""s are entrained, and by which the emission of the VOC""s from drying wafers can be advantageously controlled. The novel method reduces the RTO capacity required by a significant degree while at the same time recovering the fuel values of the VOC""s which have heretofore been lost. Finally, the need to use one or more heat exchangers to heat a drying air stream with combustion gases can be eliminated entirely. These and other aspects of the invention will now be described in greater detail by reference to the drawings.